This invention provides a method for elevating very large blocks of earth. The method may be applied in the creation of underground storage cavities, mining, and in situ hydrocarbon recovery.
A slurry mixed from locally excavated materials including a substantial percentage of clay is used to stabilize underground openings associated with the process. The use of clay slurry to stabilize underground openings has been widely practiced for many years both in rotary drilling and in construction. The use of slurry to stabilize a deep trench during its excavation is described in U.S. Pat. No. 2,757,514. A very similar method of excavation in a slurry filled trench is disclosed in U.S. Pat. No. 3,776,594 for the recovery of mineral from a steeply pitching vein, B. C. Gerwick, Civil Engineering ASCE, Dec. 1967 p. 70-72, reviews the use of slurry in stabilizing deep excavations for foundations and the like.
The present invention employs hydraulic fracture propagation for separating rock masses. Such an application for quarrying rock masses is disclosed in U.S. Pat. No. 703,302. The present invention makes novel use of a gelled slurry as a working fluid to displace extremely large blocks of earth materials. In this respect the invention is similar to a preferred embodiment of the mining method disclosed in U.S. Pat. No. 3,917,349 where massive blocks of coal overburden are displaced in a substantially horizontal direction by hydrostatic slurry pressure. The present invention is distinguished from the above embodiment of U.S. Pat. No. 3,917,349 in that the blocks of earth are displaced either vertically or steeply upward by fluid pressure. The present invention has a wide range of applications. One object is to provide an economical method for creating a deep underground cavity. The cavity may then be used for storage of liquids, or in another group of applications the cavity may provide the void needed for fragmenting coal or oil shale in preparation for the in situ recovery of hydrocarbons. In the past such preparations have been made by such methods as detonating explosives in bore holes and in hydraulic fractures. These methods have not proven satisfactory because a major portion of the deposits within the intended recovery area are not fragmented to sufficient porosity and permeability so that injected reactants fail to contact major portions of the resource, which is then by-passed and not recovered. Expensive but physically more efficient methods of preparing oil shale for in situ recovery have been proposed. By these methods openings are mined out of the shale and adjacent shale is blasted or stoped into the openings. Such proposals are made in U.S. Pat. No. 3,001,776 and 3,316,020 and 3,434,757. The resulting oil shale filled cavities are then operated as retorts. The shale is heated by partial combustion with injected air. The resulting heat of combustion distills out the oil which is drawn from wells connected to the bottom of the retort. It is an object of the present invention to provide a more economical method for producing such underground retorts filled with fragmented coal or oil shale while avoiding the expense of deep underground mining.
In yet another group of applications, the present invention is used to elevate massive sections of coal bed for recovery at the surface. In these cases the present invention provides economical alternatives to conventional mining.
In the past, recovery of coal from steeply dipping beds has often been by surface mining. This method has limited practical depth of application because the volume of overburden moved and surface area disturbed increase rapidly with depth. Mechanized underground mining of steeply dipping seams involves major problems with movement and operation of equipment and with miner safety. Accordingly, it is an object of the present invention to provide an economical method for recovery of coal to substantial depths from steeply dipping beds, that minimizes surface disturbance, and allows operators to remain at the surface. U.S. Pat. No. 3,874,733 and U.S. Pat. No. 3,776,594 disclose mining techniques for recovering coal from a nearly vertical seam where a dense slurry fills the mine cavity and serves to float fragmented coal to the surface of the cavity. The coal is then separated from the slurry and the slurry is reused. The slurry used in such a mining system must contain a high percentage of expensive weighting material such as fine iron ore. The special weighting material is needed so that the required slurry density is compatible with the other qualities needed for efficient flotation and separation of fragmented coal. For this use of such weighted slurry to be economical, loss of expensive weighting material in the mined out cavity must be kept small. But since very large volumes of slurry are required to fill the mine cavity it is very difficult to prevent excessive loss of weighting material.
When the present invention is employed to recover massive blocks of coal, these difficulties are avoided. The dense slurry used with the present invention is mixed from common earth materials excavated locally and is so inexpensive that it is economical to expend large quantities during the mining operation. This slurry is unsuitable for use in flotation and subsequent separation of fragmented coal, as proposed in U.S. Pat. Nos. 3,874,733 and 3,776,594, because of its high gel strength, viscosity, and solids content, but these qualities do not impede its use in the present invention because the large blocks of coal recovered are easily washed free of contaminating slurry.